1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus including the same.
2. Description of the Related Art
In recent years, for electrophotographic image forming apparatuses supporting full-color and high-quality images, a dual-component developer (hereinafter simply referred to as the “developer”) which exhibits an excellent charge performance stability as to a toner is in widespread use.
The developer is made up of the toner and a carrier, which are agitated in a developer vessel of a developing device and frictionally rubbed with each other to produce an appropriately electrified toner.
In the developing device, the electrified toner is supplied onto a surface of a developing roller. The toner is moved by an electrostatic attraction from the developing roller to an electrostatic latent image formed on a surface of a photoconductor drum.
Thus, a toner image based on the electrostatic latent image is formed on the photoconductor drum.
Further, there has been an increasing demand for the image forming apparatuses that operate faster and that are miniaturized, which is associated with the necessity to electrify the developer quickly and sufficiently and to convey the developer quickly.
To this end, Prior Art 1 proposes a circulative developing device, which includes first and second developer conveying passages partitioned by a partitioning plate disposed in a developer vessel, first and second communicating paths establishing a communication between the first developer conveying passage and the second developer conveying passage at opposite ends, and first and second auger screws disposed in the first and second developer conveying passages to convey a developer in directions opposite to each other (for example, see Japanese Unexamined Patent Publication No. 2001-255723).
With the developing device, the developer having been conveyed to a downstream point in the first developer conveying passage by the first auger screw is pushed by the developer conveyed from an upstream point in the first developer conveying passage, thereby to be pushed out of the first communicating path into the second developer conveying passage along an interior wall face of the developer vessel. Further, the developer having been conveyed to a downstream point in the second developer conveying passage by the second auger screw is pushed by the developer conveyed from an upstream point in the second developer conveying passage, thereby to be pushed out of the second communicating path into the first developer conveying passage along the interior wall face of the developer vessel. In this manner, the developer circulates through the first developer conveying passage and the second developer conveying passage.
However, with the developing device disclosed in Prior Art 1, because the developer is pressurized at the downstream point in the first developer conveying passage and at the downstream point in the second developer conveying passage, a stress put on the developer becomes great.
When such a stress is put on the developer for a long period, a fluidity improver which is a toner external additive is buried under a surface of the toner. This causes a phenomenon of a reduction in a fluidity of the developer, which eventually makes it difficult for the developer to be conveyed. As a result, the amount of the developer supplied to the photoconductor drum via the developing roller tends to become insufficient, which disadvantageously results in a low-density image printed on a recording medium.